At 12:38 AM 9/23/2009, Frank Dernie wrote:
>Hi Peter,
>Surely you mean the reverse?
>Film flatness has always been a potential problem, particularly with
>larger formats, and the emulsion of film is effectively "thick"
>compared to the plane on which a lens must focus for a digital
sensor.
>Digital therefore becomes much more sensitive to focus errors and
lens
>shortcomings. Film is much more forgiving.
>In case you missed it the first time I posted the link, there is an
>interesting essay on the difficulty of realising the full potential
of
>digital systems here:-
Frank
You are essentially correct. All lenses produce a spherical focal
"plane", even APO corrected "flat-field" lenses. This was a problem
with early lenses which often displayed out-of-focus areas at the
edge of the image but the improvement in lens designs and in
emulsion speeds allowed depth-of-focus to disguise the effect.
Still, it remained an issue on fast Schmidt astro telescopes, which
required the film to be held in a spherical cup.
Process lenses were probably the best-corrected for producing a
relatively flat focal plane and that was done by having them produce
a focal "plane" based on a very large sphere to minimize the issue.
Digital will have the same problem but it will have a different set
of criteria and so a lens designed for film might well not work as
well on digital, though Leica is marketing their later lenses as
compatible with both and I have not heard of any specific problems
with the longer lenses (say, 28mm and above) when used in both film
and digital cameras.
Bear in mind that I unsubscribed from the Leica Users' Group a year
or so back, so there might have been a discussion over there of
which I am unaware.
Marc
msmall@xxxxxxxxxxxx
Cha robh bàs fir gun ghràs fir!
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